The combustion of any fossil fuel requires a fixed and known quantity of combustion air required to burn any given quantity of fuel. This match between air and fuel is referred to as stoichiometric combustion conditions. It is only in theory that stoichiometric air can be supplied to the combustion and all the fuel be consumed. As a practical matter, a furnace of infinite size would be required. Therefore, more air is supplied than is theoretically required. This additional quantity is referred to as "excess, secondary" air. If excess, secondary air were not added to a standard size furnace, the substoichiometric combustion would produce flue gas with significant quantities of incomplete products of combustion which would constitute hydrocarbons, char, and carbon monoxide. The excess air eliminates these undesirable elements, but at the same time, provides O.sub.2 for the formation of nitrous oxide or NOx is a regulated pollutant. To provide a satisfactory balance between the two extreme conditions, only stoichiometric air is injected for the first parts of the combustion process and the remaining portion of the excess air is subsequently injected through overfire air (OFA) ports. A representative literature on this subject is the Leslie Pruce article "Reducing NOX Emissions At The Burner, In The Furnace, And After Combustion" appearing on pages 33-40 of the January, 1981 issue of Power.
In tangential firing, the products of combustion are forced into a rotating or swirling pattern in the furnace. This is excellent for mixing fuel and air but has several drawbacks. First, it produces a lot of horizontal gas patterns, many of which collide with the boundary waterwall and deposit ash on the walls of the furnace. Secondly, the change in direction of the swirling pattern into the non-swirl convection section causes a non-uniform pattern, and a maldistribution of temperature and mass flow across the furnace outlet plane is known as "unbalance". Unbalance leads to numerous operational and design problems which have always been considered as a "given" with this form of firing system. If the overfire, secondary excess air can be introduced to eliminate the swirl prior to entry of the flue gases into the convection section, the unbalance phenomena will be avoided without affecting the swir1 in the early part of the combustion process.